1. Field of the Invention
The present invention generally relates to a shutter blade for use in a focal plane shutter or a lens shutter of a camera and a diaphragm blade of a camera, to be more specific, to a plastic light-shielding blade for use in a camera that requires to block light.
2. Description of Related Art
A blade such as a shutter blade and a diaphragm blade for use in an optical apparatus such as a camera must be light in weight and high in rigidity and, at the same time, must intrinsically have light-shielding properties because the blade covers a photosensitive material at the front side thereof to block light from being exposed to the material. Besides, it is desired for the blade to be black in color and low in reflectivity because the blade must absorb light to prevent the light from being reflected on the surface of the blade. Further, the blade must have a good planarity and a high resistivity against contact electrification because the blade operates in contact with another. These properties are especially important for a shutter blade that operates at a high speed. To satisfy the above-mentioned requirements, various light-shielding blades have been developed as disclosed for example in Japanese Unexamined Utility Model Publication No. Sho 60-63825, Japanese Unexamined Patent Publication No. Sho 63-165828, Japanese Unexamined Patent Publication No. Sho 57-17932, Japanese Unexamined Patent Publication No. Sho 57-24925, Japanese Unexamined Patent Publication No. Sho 57-60315, Japanese Unexamined Patent Publication No. Sho 57-118226, Japanese Unexamined Patent Publication No. Sho 58-149030, Japanese Unexamined Patent Publication No. Hei 4-211232, and Japanese Unexamined Patent Publication No. Hei 7-270855. Of the above-mentioned publications, No. Sho 57-24925 discloses a light-shielding blade made of an aluminum alloy sheet for use in a camera. No. Sho 58-149030 discloses, for light-shielding blade materials, plastics such as polyethylene, polypropylene, polyvinyl chloride, polyamide, epoxy resin, phenolic resin, urea resin, polyurethane, polystyrene, polypropylene, and unsaturated polyester resin, or materials obtained by reinforcing these materials with fibers such as glass fiber, carbon fiber, boron fiber, higher polymer organic fiber, boron nitride, and whisker. No. Hei 4-211232 discloses use of aromatic polyamide fiber for a blade support arm.
Currently, a light-shielding blade made of polyethylene terephthalate (PET) is in wide use because of its low cost and light weight. However, because PET is comparatively low in strength such as tensile elasticity, PET is not suitable for use in a light-shielding blade for a focal plane shutter that operates at a high speed. If PET is used as the material of a light-shielding blade, the blade is deflected by vibration or shock caused by traveling or braking of the blade. This deflection leads to damage of the blade or collision between blades. Therefore, guaranteed durability of the shutter using PET light-shielding blades is generally in the order of 10,000 shutter operation times, far less than 30,000. Actually, if PET light-shielding blades are used for a focal plane shutter, a derived maximum shutter speed is limited to 1/1000 second to 1/2000 second. To achieve shutter speeds of 1/4000 second or higher, in case that a focal plane shutter is constituted by four light-shielding blades for example, the first and second blades are made of aluminum alloy (for example, superduralumin) and the third and fourth blades are made of PET. If all of four blades are made of PET, the deflection may be caused by traveling or braking of the blades.